There, because it prevents the black stones from having to run and if White plays a or c, Black still gets the other one, but doesn't have to fear for the life of the group, which gives him strength elsewhere on the board.
Why isn't c as good? It secures the sides and corner, though I'd rather have played it at f... and what's wrong with the black stones there running? Build up some thickness on the top to disrupt White...
Fhayashi: Because white "c" now would potentially create a weak white group in the midst of all the black stones (imagine black "f"), while white "b" would make the two black stones on top weak, without white "b" itself being weak.
ProtoDeuteric- I believe that the question to which Fhayashi responded was meant to ask why black wouldn't play "c". I agree that "b" is better, but I just want to clear up the question.
ThaddeusOlczyk: The best that the right black stone can strengthen himself is by playing at z. Yes it doesn't have far to run, but it is still weak. That means the upper group would be a second weak group. Solidify it with b.
Move on to: Fuseki Exercise 3